The present invention relates to a laser system consisting of an oscillator and laser amplifier, and using double or multiple passes in the amplifier for achieving efficient high energy amplification. More particularly, the present invention is concerned with systems and methods utilizing reflecting means including a phase conjugate mirror for achieving high energy and efficient multipassage amplification.
High power lasers are of a design including a master oscillator and a power amplifier. Usually, in order to efficiently extract the power stored in the amplifier rod, two or more amplifying passes are needed. In such a system, the beam passes through the amplifier in two directions, and polarization output coupling is required for extracting the laser beam.
FIG. 1 (prior art) illustrates a laser system with an oscillator/amplifier and a phase conjugate mirror. This known system is arranged in a manner utilizing a faraday isolator rotator 2 placed between an oscillator 4 and amplifier 6 to divert the reflected beam from the oscillator. The laser beam from oscillator 4 passes through polarizer 8, interposed between the oscillator 4 and isolator/rotator 2 as a linearly polarized beam toward the faraday isolator/rotator 2. The faraday isolator/rotator 2 rotates the angle of polarization by 45xc2x0, and a second polarizer 10, which is in position to pass the laser beam in the new direction of polarization, routes the beam toward amplifier 6. The beam, amplified as it passes through the amplifier 6, is circularly polarized by a quarter wave plate 12 and focused into a non-linear medium by a focusing lens 14 before entering a phase conjugate mirror 16. The phase conjugate mirror 16 retroreflects the input laser beam back through the lens 14 and converts it back to linear polarization but rotated 90xc2x0 with respect to incident polarization by the quarter wave plate 18. The double amplified laser beam 18 which is the output of amplifier 6 is reflected by polarizer 10 out of the laser system. The fraction of the beam that has been affected by the amplifier birefringence will pass through the polarizer to the faraday isolator/rotator 2, which rotates its linear polarization by 45xc2x0, and polarizer 8 reflects it out of the system in a different direction 20 from the direction of the output laser beam 18.
When the direction of the beam is toward the oscillator 4, there is a risk that a part of the laser beam will return and enter into the oscillator, thereby affecting the performance of the laser, or even damaging optical elements of the oscillator. This problem is even more acute when solid state lasers are used, as a result of thermal effects. At high average input powers, the non-uniform temperature distribution in the amplifier rod induces significant birefringence via thermally induced stresses. Such thermal birefringence can lead to strong depolarization of the laser radiation, so that a significant fraction of the amplified beam goes into the oscillator. In order to prevent same, an optical isolator must be inserted between the oscillator and the amplifier. The isolator diverts the fraction of the beam which has the wrong polarization away from the oscillator, but in a direction different from the original beam direction, resulting in reduction of the laser""s efficiency.
Several solutions to the thermally induced problem of birefringence have been proposed over the years, but none has ever performed reliably enough, or been deemed sufficiently practical, to receive widespread acceptance in the solid-state laser community. An example of such a solution, called the xe2x80x9cScott and dewitt scheme,xe2x80x9d is illustrated in FIG. 2.
In this prior art scheme as shown in FIG. 2, the partially depolarized beam from the first amplifier 6 passes through a rotator 22, which rotates the polarization of the beam by 90xc2x0 and passes the rotated beam through the second amplifier 24. To the extent that the two amplifier rods are identical, the birefringence induced by the rod of the first amplifier 6 is canceled by the rod of the second amplifier 24. At the output of amplifier 24, the beam is linearly polarized in a direction rotated by 90xc2x0 from the output beam of the oscillator 4. The beam polarization is made circularly by a quarter wave plate 12, and enters the phase conjugate mirror 15 via focusing lens 14. The phase conjugate mirror 16 retroreflects the input laser beam back through the lens 14. The beam is turned back to linear polarization but rotated at 90xc2x0 with respect to the incident polarization by quarter wave plate 12 and is amplified by the two amplifiers 24 and 6. The induced birefringence is reduced in the same way as in the first pass. The twice amplified laser beam output from amplifier 6 is reflected by the polarizer 10 and exits the laser system as beam 18 after it has been polarization rotated by 90xc2x0.
The above-described scheme attempts to eliminate birefringence effects by placing a 90xc2x0 rotator (plate 22) between two identical laser rods of amplifiers 6 and 24. The idea is that, to the extent that the two rods are identical, the birefringence induced by one rod is canceled by the other. However, the success of this solution varies in consideration of heating non-uniformities and other practical issues related to the requirement that the rods be identical; for example, if, for any reason, one of the amplifier""s rods has to be changed, both rods must be changed at the same time, to assure identicality. In addition, this system requires the use of two amplifiers.
It is therefore a broad object of the present invention to ameliorate the disadvantages of the prior art systems and to provide a laser amplification system and method for efficiently effecting multiple pass amplification.
It is a further object of the present invention to provide a system and method for effecting an unidirectional, multiple pass amplification of a laser beam through one or more amplifiers.
In accordance with the present invention, there is therefore provided a laser system for producing a high energy amplified laser beam output from an oscillator producing a wave front of low energy laser beam, said system comprising at least one amplifier positioned to receive said low energy laser beam for amplification via a first polarizer; a second polarizer, positioned along the axis of the amplified beam at the output side of said amplifier, for allowing a first fraction of the beam to pass therethrough and for reflecting a second fraction of said beam from an output surface of said polarizer, said second fraction constituting the output of the system; a retroreflector, associated with a quarter wave plate, oriented to receive said first fraction and to reflect it back toward said second polarizer; reflecting means for reflecting said reflected first fraction toward said first polarizer to be reflected toward said amplifier for further amplification and to be reflected off the output surface of said polarizer together with said second fraction.
The invention further provides a laser system for producing a high energy amplified laser beam output from an oscillator producing a wave front of low energy laser beam, said system comprising at least a first and a second amplifiers disposed in non-axial relationship to each other, said first amplifier being positioned to receive said low energy laser beam for amplification via a first polarizer; reflecting means for reflecting the amplified beam of said first amplifier toward the input side of said second amplifier; a second polarizer, positioned along the axis of said second amplifier at the output side thereof, for allowing a first fraction of the beam to pass therethrough and for reflecting a second fraction of said beam from an output surface of said second polarizer, said second fraction constituting the output of the system; and a retroreflector associated with a quarter wave plate, oriented to receive said first fraction and to reflect it back toward said first polarizer, to be reflected thereby toward said first amplifier for further amplification by said first and second amplifiers, and to be reflected off the output surface of said second polarizer together with said second fraction.
The invention still further provides a method for amplifying a laser beam, said method comprising directing a laser beam via a first polarizer toward the input side of at least a single amplifier, to be amplified; passing a first fraction of the amplified beam via a second polarizer toward a retroreflector including a quarter wave plate and allowing a second fraction of said beam to be reflected from an output surface of said second polarizer, said second fraction constituting the output of the system, and reflecting said first fraction toward said first polarizer via a quarter wave plate to be rotated and, in turn, to be reflected toward the input side of said amplifier for further amplification, in the same direction of amplification of said laser beam, and to be reflected off the output surface of said second polarizer together with said second fraction.
The invention yet further provides a method for amplifying a laser beam, comprising directing a laser beam to be amplified via a first polarizer toward the input side of a first amplifier; reflecting the amplified beam toward the input side of a second amplifier for further amplification; directing the further amplified beam toward a second polarizer, allowing the passing therethrough of a first fraction of said further amplified beam and reflecting a second fraction of said beam off an output surface thereof, said output surface constituting the output of the system; reflecting said first fraction back toward said second polarizer via a quarter wave plate to be rotated and, in turn, to be reflected toward the input side of said. first amplifier for still further amplification by said first and second amplifiers, and to be eventually reflected off the output surface of said second polarizer together with said second fraction.